1. Field of the Invention
The present invention relates to a thin film formed by sputtering for coating mainly on a surface of an optical element, and a process for forming the thin film.
2. Related Background Art
A film having a refractive index continuously varying, like a refraction-gradient film, is formed generally by CVD as shown in Japanese Patent Application Laid-Open Nos. 5-65648, 5-65649, 7-56001, and 9-68601. It can be formed also by a sputtering method by use of plural targets as shown by S.Yoshida and H.Yajima ("Hakumaku Hikari Devaisu (Thin-Film Optical Device)" (Tokyo Daigaku Shuppan K.K.)). An oxide film having refraction-gradient can be formed by sputtering by varying an oxygen partial pressure to control oxidation number, or by varying the film density.
A refraction-gradient film can be produced by using a fluoride as a target material, introducing an oxygen-containing gas, and varying the electric power applied to the target as shown in Japanese Patent Application Laid-Open No. 8-201601.
Ion beam sputtering can be employed with an oxide as the target as shown Japanese Patent No. 2,556,206.
In still another method, a metal film is formed by use of plural metallic targets and then the metal film is converted to a compound as shown in Japanese Patent Application Laid-Open No. 9-263937.
Conventionally, a film having a refractive index varying continuously, like refraction-gradient film, is formed from combination of an oxide material having an intermediate refractive index, and a fluoride material or a nitride material.
Gases containing a metal element employed generally in CVD are dangerous because of their flammability, corrosiveness, toxicity, and so forth, which causes high cost for the gas supply system and the gas treatment system. The film formed at a low base-plate temperature (not higher than 200.degree. C.) will be affected adversely by impurity of the gas component to have lower density, lower adhesiveness, lower strength, and poor optical properties.
In formation of a laminated film in which a refraction-gradient film is one of the constituting layers, practice of CVD only for the refraction-gradient film is not efficient.
In sputtering with plural kinds of targets, the respective targets of different sputtering yields should be controlled separately to control the refractive index of the formed film on the base plate. This process for controlling arbitrarily the refractive index of the film is complicated due to the need to control the applied electric power, partial gas pressure, distribution of sputtered particles, and so forth.
Reactive sputtering is more complicated because of the difference of reactivity between the target materials.
The method in which the degree of the oxidation of oxidation film is varied by the oxygen partial pressure to vary the refractive index is limited to an applicable wavelength region because of the dependency of the light absorption on the oxidation degree.
The method in which the film density is varied poses problems in weatherability, if the density is lower, such as variation of the refractive index with lapse of time by moisture absorption, and burning of the base plate by penetration of moisture through the film.
The method disclosed in Japanese Patent Application Laid-Open No. 8-201601, in which applied electric power to a fluoride target is varied with the rate of introduced oxygen-containing gas kept unchanged, varies the oxidation degree by varying the film-forming rate. The productivity of this method becomes extremely low when obtaining a film of high oxidation degree.
The method disclosed in Japanese Patent 2,556,206, in which a dielectric material such as an oxide or a fluoride is used as the target material, has the disadvantage that the purity of the target cannot readily be raised and the impurity contaminates the film, which affects adversely the optical properties such as light absorption of the film. Further, the dielectric material target is expensive. When a granular target is employed, the sputtered particles form clusters in the film depending on the size of the granules, which varies the refractive index of the film, the density, surface state, and internal structure of the film to lower the film quality in the film forming process. Furthermore, DC current cannot be effectively applied to the dielectric target to lower the rate and limit the arbitrariness in the process.
The apparatus provided with plural targets of different metal materials in a reaction chamber, as shown in Japanese Patent Laid-Open No. 9-263937, has a complicated structure including a base plate delivery mechanism, and is expensive. With such an apparatus, two or more treatment steps are necessary for obtaining a simple film of one refractive index since a metal film is formed once and is converted to a metal compound.
With increase of the film thickness, as is expected, the necessary for the film formation increase, and the stress in the film increases to lower the durability of the film.
Conventionally, a film having a refractive index varying continuously, like a refraction-gradient film, is formed from combination of an oxide material having an intermediate refractive index, and a fluoride material or a nitride material. The refractive index variation is limited to be in a narrow range between the higher refractive index and the intermediate refractive index, or between the intermediate refractive index and the lower refractive index.